Vestibular Implant System with Internal and External Motion Sensors

ABSTRACT

A vestibular prosthesis system is described which includes an external movement sensor that is attachable a patient&#39;s head for generating an external movement signal. An internal movement sensor is implantable under the skin of the patient&#39;s head for generating an internal movement signal. A fail-safe sensor is configured to detect movement of the one or more external movement sensors relative to the head and generate a corresponding relative motion signal. And an implant processor also is implantable under the skin and in communication with the internal movement sensor and the external transmitter for generating an implant stimulation signal based on one of the movement signals and/or the relative motion signal to electrically stimulate target neural tissue for vestibular sensation by the patient.

This application is a continuation of U.S. patent application Ser. No.13/187,979, filed Jul. 21, 2011, which in turn claims priority from U.S.Provisional Patent Application 61/366,345, filed Jul. 21, 2010, which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to implantable stimulation systems, andmore specifically to a vestibular implant system with internal andexternal motion sensors.

BACKGROUND ART

A normal ear directs sounds as shown in FIG. 1 from the outer ear pinna101 through the generally cylindrical ear canal 110 (typically about 26mm long and 7 mm in diameter) to vibrate the tympanic membrane 102(eardrum). The tympanic membrane 102 moves the bones of the middle ear103 (malleus, incus, and stapes) that vibrate the cochlea 104, which inturn functions as a transducer to generate electric pulses to the brainthat are interpreted as sounds. In addition, the inner ear also includesa balance sensing vestibular system which involves the vestibularlabyrinth 105, its three interconnected and mutually orthogonalsemi-circular canals: the superior canal 106, posterior canal 107, andhorizontal canal 108 (as well as the otolith organs, the utricle andsaccule—not shown). The canals and spaces of the vestibular labyrinth105 are filled with endolymph fluid which moves relative to headmovements, thereby activating hair cells that send an electrical balancesignal to the brain via the vestibular nerve 111.

In some people, the vestibular system is damaged or impaired, causingbalance problems such as unsteadiness, vertigo and unsteady vision.Vestibular implants are currently under development, with one of theinitial challenges being the relatively significant amount of powerrequired by the gyroscope/accelerometer arrays used for the movementsensors (gyroscopes and linear accelerometers). Presently, one lowerpower device is the STMicroelectronics L3G4200D, which is a three-axisdigital gyroscope that encodes all three-dimensional axes of rotation.This device measures 4 mm square by 1 mm thick and needs various powerand signal lines to operate at the specified power requirement of atleast 6 mA at 3.6V.

For use in an implant system, the power for one or more vestibularmovement sensors can be supplied from a body worn battery pack andtranscutaneously transmitted with a head placed coil. But the powerlosses for such a transcutaneous supply are roughly a factor of two andthere also is an additional risk of the head coil falling off, powerbeing lost, and the patient becoming disoriented and even falling. Asomewhat better solution might be to have an implanted battery supplyingpower to the implanted movement sensors, but (due to the high powerrequirements) this approach is likely to require a large battery volumeor very frequent re-charging of the battery. Furthermore, failure of anyof the modules of the device would require re-implantation, withconsequent risk to hearing and residual vestibular function.

Other arrangements have also been proposed for vestibular implantsystems. For example, head-worn sensor arrays have been proposed thatwould be secured by a holding band around the head, but this approachwould create an unacceptably high risk of movement of the sensorsrelative to the head. Implanted sensor arrays powered via a percutaneousplug have also been proposed, but the serious problems with percutaneousstructures are notoriously well-known. Challa and Bhatti, AMicromachined Cupula: Toward Biomimetic Angular Velocity SensorProsthesis, 33 ^(rd) Mid-Winter Research Meeting, Assn. for Research inOtolarygology, Feb. 6-10, 2010 (incorporated herein by reference)proposed a basic re-design of the sensor array around the fluidicprinciple used by the balance organ itself to reduce power requirements,but more time and extensive reliability testing will be needed tocomplete the development of such a device.

Application US2005/0267549 by Della Santina et al. (incorporated hereinby reference) teaches a combined cochlea/vestibular stimulation systemwith a speech and motion sensing processor (SMP) placed eitherexternally or internally. Application US2002/0104971 by Merfeld et al.(incorporated herein by reference) teaches a motion sensing system to beworn not only on the head but also on other body parts.

For safety reasons, it is important that the externally worn unitincluding the sensor is always placed in a known, correct orientationwhen driving the implant. Otherwise the sensor's misaligned input tosignal processing, and ultimately to the neural stimulation sites, willlead to a mismatch between real and perceived head movement. Underspecific circumstances this may cause a patient to fall and possiblyresult in injury. This is of special relevance when the implant islocated on the head such that sometimes the patient cannot visuallyobserve placing the external unit over the implant.

The correct placement of an external unit relative to an implant iscurrently solved for cochlear implants and other auditory implants by apair of axially magnetized magnets. One magnet is placed in the centerof the implant's receiver coil. The other magnet is placed in the centerof the sender coil in the external unit. While placing the externalunit's magnet in proximity to that of the implant, the magneticattraction force causes the external coil to be placed over theimplant's coil in a concentric orientation. But there is a remainingdegree of freedom in that the external unit can be turned in the radialdirection a full 360 degrees relative to the implant. Due to this degreeof radial rotation freedom, this solution is not appropriate for placingan external sensor as part of a vestibular implant system.

SUMMARY

Embodiments of the present invention are directed to a partiallyimplantable vestibular prosthesis system which includes an externalmovement sensor module that is attachable to the outer skin surface of apatient's head for generating an external movement signal whichrepresents movement of the patient's head. An external transmitter is incommunication with the external movement sensor and provides anelectromagnetic transmission of an implant communication signal whichincludes a signal component based on the external movement signal and anelectrical power component that provides electrical power for theimplanted system components. An additional internal movement sensor isimplantable under the skin of the patient's head for generating aninternal movement signal which represents movement of the patient'shead. A fail-safe sensor is configured to detect movement of the one ormore external movement sensors relative to the head and generate acorresponding relative motion signal. And an implant processor also isimplantable under the skin and in communication with the internalmovement sensor and the external transmitter for generating an implantstimulation signal based on one or more of the movement signals and/orthe relative motion signal to electrically stimulate target neuraltissue such as the semicircular canals of the inner ear, the otolithorgans, and/or the vestibular nerve for augmentation or modification ofthe patient's vestibular function.

In some specific embodiments, the external movement sensor may beadapted to be attachable within the ear canal of the patient's head. Forexample, the external movement sensor module may be magneticallyattachable within the ear canal over a corresponding implanted holdingmagnet which establishes a correct position for the external movementsensor. The external movement sensor module also may be adapted to leavea portion of the ear canal unoccluded to allow hearing sensation by thetympanic membrane. In other embodiments, the external movement sensormodule may be magnetically attachable on the scalp of the patient's headover a corresponding implanted holding magnet which establishes acorrect position for the external movement sensor module.

For example, the external movement sensor module may include multiplemagnets for cooperation with corresponding multiple magnets in theimplant magnet arrangement. In such an embodiments, the magnets in theexternal movement sensor may be arranged with asymmetrical magneticpolarities, with spatial asymmetry, with asymmetrically differentdiametrical orientations, and/or with asymmetrically different distancesbetween cooperating pairs of magnets. And the magnets in the externalmovement sensor may be arranged to require orientation of the externalmovement sensor so that each magnet in the external movement sensor iscooperating with another corresponding magnet in the implant magnetarrangement in order to have enough magnetic attraction force tosecurely hold the external movement sensor.

The implant stimulation signal may be preferentially based on theexternal movement signal when available, and otherwise may be based onthe internal movement signal. In addition or alternatively, the implantstimulation signal may be preferentially based on the internal movementsignal when the external movement signal exceeds some acceptablethreshold value or when communication between the external and internalcomponents transfers control. The implant processor may include abaseline pacing mode wherein the implant stimulation signal is generatedwithout reference to a movement signal.

Some embodiments may also include a sensor position sensor forgenerating a sensor movement signal representing movement of theexternal sensor relative to the patient's head.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows various anatomical structures associated with the humanear.

FIG. 2 shows one specific example of a partially implantable vestibularprosthesis system.

FIG. 3 shows one arrangement for an implant processor for use inembodiments of the present invention.

FIG. 4 shows some of the external components of one specific embodiment.

FIG. 5 shows a top view of various components associated with a scalpattached external movement sensor.

FIG. 6 shows a cross-sectional view of one specific scalp-attachableexternal movement sensor arrangement using two attachment magnets.

FIG. 7 shows an embodiment of an external movement sensor using threeattachment magnets.

FIG. 8 shows a sectorized attachment magnet for use in embodiments ofthe present invention.

FIG. 9 A-D shows placement details of an embodiment of an externalmovement sensor using three symmetrical attachment magnets.

FIG. 10 A-D shows placement details of an embodiment of an externalmovement sensor using three asymmetrical attachment magnets.

DETAILED DESCRIPTION

Embodiments of the present invention are directed to a safe, practicableand wearable partially implantable vestibular prosthesis system whichcombines both internal and external movement sensors. By makingappropriate design choices, many of the drawbacks from earlierapproaches can be avoided. For example, in some embodiments the internalimplant movement sensor can be mainly used as a backup signal source fora few hours (depending on the battery life of the implant battery) whenthe main externally worn movement sensor is unavailable or unreliable.Other embodiments may use an implanted movement sensor as its normalmain signal source, with an external movement sensor serving as a systembackup signal source in the event of failure by the implanted movementsensor.

FIG. 2 shows one specific example of a partially implantable vestibularprosthesis system with an external movement sensor 208 that includes athree-axis digital gyroscope array 210 that generates an externalmovement signal encoding three-dimensional axes of rotation representingmovement of the user's head. The external movement sensor 208 is adaptedto be positioned within the ear canal 110 of the user where anattachment magnet 209 within the external movement sensor 208 cooperateswith a corresponding implanted holding magnet 207 that is surgicallyimplanted under the skin of the ear canal 110.

An external movement sensor 208 that fits in the ear canal 110 also iswell-suited for mild to moderately deaf patients (who make up quite alarge number of patients with vestibular problems) and might beincorporated together with a hearing implant, or it might be matchedwith hearing or a hearing aid on the other side, and/or might include asound channel or large vent to partially transmit sound for patientswith hearing in that ear. Locating the external movement sensor 208 deepin the bony section of the ear canal 110 provides a low probability ofmany movement artifacts and a low risk of being displaced by normalactivities. This is in contrast to a more shallow location in the loweranterior portion of the ear canal 110 where jaw movements may causeunacceptable movement artifacts. An ideal location for external movementsensor 208 may be as shown in FIG. 2, fixed in place (e.g., with aretaining magnet) adjacent to the posterior, superior potion of thecanal wall, deep in the bony section of the ear canal 110. Adverseeffects of jaw movements will be minimized using this location and maybe further minimized through use of a partially occluding design or aflexible occluding volume.

For hearing patients, the external movement sensor 208 normally shouldtake up as little room as possible in the ear canal 110 to allowadequate transmission of sound to the tympanic membrane 102. It may beadvantageous if the external movement sensor 208 is adapted to leave aportion of the ear canal 110 unoccluded as shown in FIG. 2 to allowrelatively normal hearing sensation by the tympanic membrane 102.

Alternatively, the external movement sensor 208 could be incorporatedinto a very deep canal fitting device such as a Lyric-type hearing aiddevice which fits deeply within the ear canal 110 nearer the tympanicmembrane 102. For hearing patients, the option of a deep canal fittingmay be especially suitable if designed to allow adequate transmission ofsounds to the tympanic membrane 102.

In another embodiment, some portion of the external movement sensor 208would need to be fitted snugly deep into the ear canal 110 forstability, but the external end of the ear canal 110 could still beavailable for other uses such as other system components. Additionaldevice-related components then could be connected to the deeply fittedpart by a very flexible cable. For example, power supply and signalprocessing components also could be located within the outer portion ofthe ear canal 110 and be connected to the external movement sensor 208by a very flexible connector.

The external movement sensor 208 is connected by a connector lead 212 toa behind-the-ear external processor 201. It is advantageous if theconnector lead 212 is very flexible so that small movements of the outerear 101 and/or the external processor 201 (such as for programming) donot disturb the fit and position of the external movement sensor 208 inthe ear canal 110. A handling member 211 also projects out of theexternal movement sensor 208 to aid the user's ability to remove theexternal movement sensor 208, e.g., for bathing or device maintenance.

The external processor 201 in turn processes the external movementsignal to generate an implant communication signal that includes asignal component based on the external movement signal and an electricalpower component that provides electrical power for implanted systemcomponents (e.g. from a battery arrangement within the externalprocessor 201). The implant communication signal from the externalprocessor 201 is transmitted across the skin by an external transmittercoil 202 to a corresponding implant receiver coil 203. FIG. 4 shows theappearance of the external components in another similar embodimentwhere an external signal processor 403 and external battery power supply402 hook onto the outer ear of the user, with a flexible cable 404connecting to an external motion sensor arrangement in the ear canal andan external transmitter coil 401 magnetically positioned on the scalp ofthe user over a corresponding implanted receiver coil.

An implant processor 206 is implanted under the skin and coupled to theimplant receiver coil 203. The implant processor 206 contains aninternal movement sensor that generates an internal movement signalrepresenting movement of the patient's head. The implant processor 206processes the implant communication signal to extract its signalcomponent (from the external movement sensor 208) and also has availablethe internal movement signal from the internal movement sensor. Fromthese, the implant processor 206 generates an implant stimulation signalbased on one of the movement signals to vestibular stimulator electrodes205 that electrically stimulate target neural tissue such as thesemicircular canals 106, 107, 108 of the vestibular labyrinth 105, oneor both otolith organs and/or the vestibular nerve 111 or ganglion forvestibular sensation by the patient as a balance signal.

FIG. 3 shows structural details of an implant housing 300 which includesa receiver coil 203 and an implant processor 206. A custom ASIC 301 inthe implant processor 206 provides the processing functionality forproducing the implant stimulation signal, e.g., sequences of multiphasicpulses of varying amplitude/duration and/or repetition frequency usingbipolar or monopolar stimulation, using single channel or multi-channelstimulation to the vestibular stimulation electrodes 305 that areconnected to the implant housing 300. The implant processor 206 maypreferentially base the implant stimulation signal on the externalmovement signal when it is available, and otherwise may use the internalmovement signal. Or in some embodiments, the implant processor 206 maypreferentially base the implant stimulation signal on the internalmovement signal when the external movement signal exceeds someacceptable threshold value. In some embodiments, the implant processor206 also may include a baseline pacing mode wherein the implantstimulation signal is generated without reference to a movement signal.The embodiment shown in FIG. 3 also has a separate reference electrodelead 304 that is useful for the case of monopolar stimulation pulses.Also contained within the implant housing 300 shown in FIG. 3 is theimplant power supply 302 (e.g., a rechargeable battery) that extractsthe power component from the received implant communications signal, andpowers the implant when the external components are not attached.

The implant housing 300 also contains the internal movement sensor 303,for example, a digital gyroscope array. The internal movement sensor 303may require less rotational sensitivity (in terms of number of degreesper second) than the external movement sensor, and therefore, may alsoneed less power. Consequently, the system power source (either in theexternal portion or an implantable battery) could operate the system fora longer time until recharging or change of batteries is required.

Embodiments are not limited to locating the external movement sensorwithin the ear canal. For example, some embodiments may be based on abutton housing processor that is positioned and stabilized on the scalpof the user in a fixed desired position over the implanted device wherethe risk of being moved or falling off is relatively low. This suggestsa button housing device 600 such as the embodiment shown in FIG. 6 thatcontains a disposable or rechargeable battery or battery array 601, atransmitter coil 604, one or more external movement sensor devices 602(e.g., gyroscope arrays), an external signal processor 605 and apositioning arrangement 603 (e.g., an external holding magnet thatcooperates with an implanted positioning magnet). The embodiment shownin FIG. 6 also includes a pressure operated control switch 606 which theuser can use to control operation of the button housing device 600and/or which can also switch an in the ear device such as thosedescribed above. Unlike the earlier in the ear canal arrangements, abutton housing device 600 as shown in FIG. 6 requires no cable and maytherefore have less chance of being displaced in normal usage.

In button housing type embodiments, the implanted device can usefullyinclude a rechargeable implant battery that can provide the implantedcomponents with enough electrical energy for at least some minutes ofoperation so that the external button housing device 600 can be takenoff for maintenance such as exchanging the battery, or for showering,etc. In some embodiments, the battery for the external button housingdevice 600 may be placed in a separate housing which is connected by acable to the external button housing device 600. In some embodiments,the external button housing device 600 may have a rechargeable batterythat can be recharged via an additional charger coil which istemporarily placed over the device (e.g., for about one hour) eitherduring normal system operation or with a special recharge mode duringwhich time the implant uses its own batteries.

The embodiment shown in FIG. 6 is based on use of a single externalholding magnet, but other embodiments such as the one shown in FIG. 5may usefully include an additional secondary retaining magnet for bothimplanted part and processor such as is described in U.S. Pat. No.6,348,070 (incorporated herein by reference). In FIG. 5, the externaldevice housing 501 includes the usual main holding magnet 503 centeredwithin an external transmitter coil 505 which cooperates with acorresponding implant holding magnet 508 centered within an implantreceiver coil 506 in an implant device 502 which also includes animplant processor and (optional) movement sensors 507. Offset from themain arrangement in the external device housing 501 is another secondaryholding magnet 504 that cooperates with a corresponding implantsecondary magnet 509 to provide an increased strength fixation of theexternal device housing 501 in a desired position over the implantdevice 502. The magnetic arrangement shown also reduces the likelihoodof rotation of the external device housing 501 to the user's head.

FIG. 7 shows another embodiment of an external device 700 having adevice housing 701 that includes a triangular arrangement of threeexternal holding magnets 702 that cooperate with a correspondingtriangular holding magnet arrangement in the implanted device. Thistriangular arrangement of three external holding magnets 702 provides avery strong and stable magnetic fixation of the external device 700 overthe implant device that also reliably prevents rotation of the devicerelative to the head. Such embodiments also provide a higher inductivelink coefficient of power since there is no magnet in the center of thetransmitter coil 704 which can absorb energy. An electronics package 705within the device housing 701 includes one or more external movementsensors, a signal processor device and power supply. It may beadvantageous if the device base 703 is made of a soft material that isbio-compatible and conforms easily to attach to the curved surface ofthe user's scalp.

Introducing some form of asymmetry into the arrangement of the externalholding magnets 702 of the external device 700 can help ensure that theexternal device 700 is held in a correct position over the implant. Forexample, FIG. 9 A-D shows placement details of an embodiment of anexternal device 700 using three external holding magnets 702 arranged inan equilateral triangle with an asymmetry of the magnet axial polarities(north vs. south). In FIG. 9A, the left hand and bottom external holdingmagnets 702 are shown with south-north outward magnetic direction, whilethe right hand external holding magnet 702 is shown with a north-southoutward magnetic direction. In FIG. 9A, the external device 700 iscorrectly oriented over the implanted receiver coil 203 and implantprocessor 206 to allow proper coupling of the communications signal intothe implant. FIG. 9B shows an incorrect attempt to place the externaldevice 700 only a single pair holding magnets are attracting and twopairs of magnets are not magnetically engaged. In this orientation,there will be no coupling of the communications signal and the forceholding the external device 700 next to the skin will be far weaker thannormal so that the user should be able to detect the improperengagement. FIG. 9C shows another incorrect placement attempt where theexternal device 700 is properly centered over the implant receiver coil203 to allow coupling of the communications signal, but only onemagnetic pair is engaged with magnetic attraction and the other twopairs are repelling—this improper orientation requires manual force tohold the external device in position, and again, the user will know thatis wrong. FIG. 9D shows yet another type of incorrect placement attemptof the external device 700 with two magnetic pairs attracting and onepair not magnetically engaged—again, there will be no coupling of thecommunications signal. Thus the asymmetry of magnetic polarities ensuresthat the user will easily detect whether or not the external device 700is correctly oriented and properly held in place.

Besides using different axial orientations of magnetic polarities (northpole facing up vs. facing down) asymmetric arranging of pairs of holdingmagnets can also be achieved in other ways such as by:

-   -   using different diametrical orientations of holding magnets    -   using different distances between any two pairs of holding        magnets    -   using holding magnets of low magnetic attraction force thus        requiring more than one pair being correctly placed to each        other to hold the external over the implant. For example, FIG.        10 A-D shows placement details of an embodiment of an external        movement sensor using three external magnets 702 with identical        axial magnetic polarities arranged in an irregular triangle,        i.e., with spatial asymmetry.

FIG. 8 shows another arrangement useful for a holding magnet in any ofthe above embodiments. That is, holding magnet 800 is divided intomultiple different magnetic sectors 801 such that adjacent sectors aremagnetized differently. In the example shown in FIG. 8, the magneticsectors 801 are in the specific form of pie shaped segments but otherspecific shapes may also be useful. Again the advantage of such anarrangement is to help avoid relative movements or rotations of theexternal device relative to the head of the patient which could generateinaccurate movement sensor signals.

As explained above, the sensor elements in the external and internalmovement sensors might typically be digital gyroscope arrays or digitalaccelerometers such as MEMS accelerometers. Small movements such as fromthe user pulling out or pushing in the external device or by unintendedmovement within the ear canal (e.g., small slippage, rotation, jawmovements) can compensated for by the location and effectiveness of theholding magnet arrangement in restoring the correct position of theexternal device. Some embodiments may also use a fail-safe sensor devicefor detecting small movements either relative to the head, or to apassive component located in the ear canal (e.g., magnet or small metalplate), or to the implant itself When activated by such relativemovement this fail-safe sensor device could either:

-   -   (a) Signal the implant to revert to a baseline pacing mode of        operation to which the patient is acclimatized,    -   (b) Switch the implant off,    -   (c) Apply a correction signal to the implant for small movements        relative to the body which would be modify (e.g., be subtracted        from) the gyroscope-based sensor movement signal until the        relative movement became too large to correct for, and/or    -   (d) Excessive movement of the external movement sensor device        relative to the body would signal to the implant to temporarily        revert to its internal movement sensor device.

The sensing elements may be based on any one of several known sensingmethodologies such as electromagnetic, optical or Hall effect sensing.For example, an electromagnetic field sensing arrangement may sense theelectric field in a search coil based on proximity to a small implantedcoil or other metal piece while a transmitter coil generates a sensingfield (like in a metal detector or eddy current sensing).

Or a movement sensing arrangement may be based on a transcutaneous lighttransmission system where a light source (having a frequency thattransmits well through body tissue and the materials of the device)would be directed at the implanted magnet (or a non-magnetic reflector)that would be coated with a surface of high reflectance. Movement of thelight source relative to the magnet/reflector would change the amount ofreflected light thereby signaling movements in the device's positionrelative to the implanted plate.

Or embodiments may have movement sensors using Hall Effect sensing.During the implantation procedure, the surgeon can implant a secondsmall permanent sensor magnet under the skin deep in the ear canal.Then, a two- or three-axis system of Hall Effect sensors could detectchanges in the exact position of the sensor magnet relative to thedevice, and thereby the position of the gyroscope relative to the head.In some embodiments, a single micro three-axis sensor that works at alow enough current might be used to make corrections to the movementsignal applied to the implant. The sensor array would be placed in alocation of relatively high field gradient, for example, to one side ofthe implanted magnet.

Capacitive sensing could also provide the basis for the movement sensorelements. The portion of the external motion sensor inserted into theear canal could contain a magnet to align it over an implanted magnet,and a capacitive sensor could then be used to indicate proximity of thedevice to the magnet and detect displacement of the device (up to about2 mm) This might be useful as either a switch or as a measure ofrelative position and/or speed/acceleration of changes in relativeposition.

Some embodiments may also include a magnetically activated mechanicalswitch (like a magnetic reed switch) to deactivate power to one or moresystem components when the ear canal device is removed, or to switch thesystem to control of the (optional) implanted movement sensors or to abackup (pacing) mode. Some embodiments also may avoid continuous highsensor currents by using a power control switch activated by movementrelative to an implanted magnet (e.g., using eddy current or capacitivesensing). The movement could then be tracked by a three-axis Hall Effect(or other) sensor arrangement so that power to monitor the relativemovements would only be needed once movement was initiated. A similarrelative movement sensing arrangement might also be useful in otherlocations around the head, such as an arrangement having magnet-basedfixation somewhere external to the ear canal. Or an inductive sensingcircuit could measure the frequency of an inductive link which changeswhen the external part is moved or removed.

Some embodiments may further include a sensor position sensor forgenerating a sensor movement signal representing movement of theexternal sensor relative to the patient's head. The system may use thisinformation to modify one or more of the other movement signals thatencode head movements. For example, this may be useful during power-upof the internal sensor as control is transferred, or for modes using astand-alone external sensor.

Embodiments of the present invention such as those described above canbe developed based on existing technologies and components. Moreover,the external components such as the gyroscope sensors can be readilyaccommodated. The overall system is also robust against failure of someof the internal components. In applications as a vestibular stimulationdevice only without additional hearing functionality, the externalcomponents would not need an opening for a microphone. As a result, theexternal components can be sealed to be waterproof (or at least morewater resistant). In addition, there is design-inherent safety bypreventing implant stimulation if the external unit is not orientedcorrectly on top of the implant.

Embodiments of the invention may be implemented in part in anyconventional computer programming language. For example, preferredembodiments may be implemented in a procedural programming language(e.g., “C”) or an object oriented programming language (e.g., “C++”,Python). Alternative embodiments of the invention may be implemented aspre-programmed hardware elements, other related components, or as acombination of hardware and software components.

Embodiments can be implemented in part as a computer program product foruse with a computer system. Such implementation may include a series ofcomputer instructions fixed either on a tangible medium, such as acomputer readable medium (e.g., a diskette, CD-ROM, ROM, or fixed disk)or transmittable to a computer system, via a modem or other interfacedevice, such as a communications adapter connected to a network over amedium. The medium may be either a tangible medium (e.g., optical oranalog communications lines) or a medium implemented with wirelesstechniques (e.g., microwave, infrared or other transmission techniques).The series of computer instructions embodies all or part of thefunctionality previously described herein with respect to the system.Those skilled in the art should appreciate that such computerinstructions can be written in a number of programming languages for usewith many computer architectures or operating systems. Furthermore, suchinstructions may be stored in any memory device, such as semiconductor,magnetic, optical or other memory devices, and may be transmitted usingany communications technology, such as optical, infrared, microwave, orother transmission technologies. It is expected that such a computerprogram product may be distributed as a removable medium withaccompanying printed or electronic documentation (e.g., shrink wrappedsoftware), preloaded with a computer system (e.g., on system ROM orfixed disk), or distributed from a server or electronic bulletin boardover the network (e.g., the Internet or World Wide Web).

Although various exemplary embodiments of the invention have beendisclosed, it should be apparent to those skilled in the art thatvarious changes and modifications can be made which will achieve some ofthe advantages of the invention without departing from the true scope ofthe invention.

What is claimed is:
 1. A partially implantable vestibular prosthesissystem comprising: one or more external movement sensors attachable tothe skin of a patient's head and configured to generate an externalmovement signal representing movement of the patient's head; an externaltransmitter in communication with the external movement sensorconfigured for transcutaneous transmission of an implant communicationsignal having an electrical power component and a signal component basedon the external movement signal; one or more internal movement sensorsimplantable under the skin of the patient's head configured to generatean internal movement signal representing movement of the patient's head;a fail-safe sensor configured to detect movement of the one or moreexternal movement sensors relative to the head and generate acorresponding relative motion signal; and an implant processorimplantable under the skin and in communication with the fail-safesensor, the internal movement sensor, and the external transmitter andconfigured to generate an implant stimulation signal based on one of themovement signals and/or the relative motion signal to electricallystimulate target neural tissue for vestibular sensation by the patient.2. A system according to claim 1, wherein the one or more externalmovement sensors includes an ear canal external movement sensorconfigured to be attachable within the ear canal of the patient's head.3. A system according to claim 2, wherein the ear canal externalmovement sensor is configured to be magnetically attachable within theear canal over a corresponding implanted holding magnet whichestablishes a correct position for the external movement sensor.
 4. Asystem according to claim 2, wherein the ear canal external movementsensor is configured to leave a portion of the ear canal unoccluded toallow hearing sensation by the tympanic membrane.
 5. A system accordingto claim 1, wherein the one or more external movement sensors isconfigured to be magnetically attachable on the scalp of the patient'shead over a corresponding implant magnet arrangement which establishes acorrect position for the external movement sensor.
 6. A system accordingto claim 5, wherein the one or more external movement sensors includes aplurality of magnets for cooperation with a corresponding plurality ofmagnets in the implant magnet arrangement.
 7. A system according toclaim 6, wherein the plurality of magnets in the one or more externalmovement sensor are arranged with asymmetrical magnetic polarities.
 8. Asystem according to claim 6, wherein the plurality of magnets in the oneor more external movement sensors are arranged with spatial asymmetry.9. A system according to claim 6, wherein the plurality of magnets inthe one or more external movement sensors are arranged withasymmetrically different diametrical orientations.
 10. A systemaccording to claim 6, wherein the plurality of magnets in the one ormore external movement sensors are arranged with asymmetricallydifferent distances between cooperating pairs of magnets.
 11. A systemaccording to claim 6, wherein the plurality of magnets in the one ormore external movement sensors are arranged to require orientation ofthe external movement sensor so that each magnet in the externalmovement sensor is cooperating with another corresponding magnet in theimplant magnet arrangement in order to have enough magnetic attractionforce to securely hold the external movement sensor.
 12. A systemaccording to claim 1, wherein the implant stimulation signal ispreferentially based on the external movement signal when available andotherwise is based on the internal movement signal.
 13. A systemaccording to claim 1, wherein the implant stimulation signal ispreferentially based on the internal movement signal when the externalmovement signal exceeds some acceptable threshold value.
 14. A systemaccording to claim 1, wherein the fail-safe sensor comprises a sensorposition sensor configured to generate a sensor movement signalrepresenting movement of the external sensor relative to the patient'shead.
 15. A system according to claim 1, wherein the implant processorincludes a baseline pacing mode wherein the implant stimulation signalis generated without reference to a movement signal.
 16. A systemaccording to claim 1, wherein the target neural tissue includes thesemicircular canals of the inner ear.
 17. A system according to claim 1,wherein the target neural tissue includes the vestibular nerve.